1. Field of the Invention
The present invention relates to a flux used when soldering various electronic components onto a circuit board, and more particularly to a flux used when soldering is conducted onto copper lands having electroless nickel plating or further having gold plating on the electroless nickel plating, as well as a circuit board.
2. Description of Related Art
Traditionally, when electronic components are generally soldered onto a printed circuit board, soldering to copper lands formed on the printed circuit board has been done by using tin/lead alloy solder and lead-free solder. Generally, to prevent copper oxidation, electroless nickel plating to the surfaces of the lands is conducted in advance, and gold plating to the surface is further conducted in advance by using a nickel plating layer as a barrier layer.
However, since hypophosphite is used as reducing agent in the electroless nickel plating to the above-mentioned lands, a trace quantity of phosphorous is contained in a nickel plating coat. Therefore, when soldering with the use of solder alloy is conducted to the surface of the electroless nickel plating or the surface having the gold plating on the electroless nickel plating, gold and the nickel in the nickel plating diffuse into the melted solder alloy. Then, at the boundaries between the nickel plating layer and the solder alloy, phosphorus segregates locally thereby to produce portions at which phosphorous is extremely concentrated, and in some cases bonding strength is lowered to strip soldering.
As a method for preventing such nickel segregation, the present applicant previously proposed a method of adding various metallic salts into flux (Japanese Patent Laid-Open No. 2003-236695). Specifically, inclusion of metallic salt in flux permits the metal in the metallic salt to be substituted by nickel and deposited. This suppresses the reaction of nickel with the metal in solder alloy, thereby retarding the diffusion of nickel in the surfaces of the lands into the solder alloy.
Unfortunately, when the salts of silver and copper are used as additives to flux, silver or copper is liable to liberate and deposit independently. Therefore, for example, in a soldering method including the steps of printing flux overall onto a substrate having lands, and placing solder balls on the lands and then having the solder balls reflow so as to connect them to the lands, metal deposits from the printed flux and forms a thin metallic film between the lands, Since the metallic film cannot be removed by cleaning, there arises the problem of impairing electrical insulating properties between the lands.